SECTION : Electrostatics

SUBJECT:
Charge Moving in a Constant Field

DESCRIPTION:

An example of the stresses transmitted by electric
fields, and of the exchange of energy between fields
and particles. An electric charge with charge q >
0 moves in a constant electric field. The charge is
initially moving upward along the negative z-axis in
a constant background field . The charge feels a constant
downward force . The charge eventually comes to rest
at the origin, and then moves back down the negative
z-axis. This motion, and the fields that accompany it,
are shown in the animation.

As the charge moves upward, it is apparent in the animation
that the electric field lines are generally compressed
above the charge and stretched below the charge. This
field configuration enables the transmission of a downward
force to the moving charge we can see as well as an
upward force to the charges that produce the constant
field, which we cannot see. The overall appearance of
the upward motion of the charge through the electric
field is that of a point being forced into a resisting
medium, with stresses arising in that medium as a result
of that encroachment.

Faraday would have described the downward force on
the charge as follows: Surround the charge by an imaginary
sphere centered on it. The field lines piercing the
lower half of the sphere transmit a tension that is
parallel to the field. This is a stress pulling downward
on the charge from below. The field lines draped over
the top of the imaginary sphere transmit a pressure
perpendicular to themselves. This is a stress pushing
down on the charge from above. The total effect of these
stresses is a net downward force on the charge.